1. Field of the Invention
The present invention relates to a motor unit fitted with a buffer mechanism, and more particularly, to a buffer mechanism for absorbing a turning force of the motor when an output system of the motor stops while the motor is still operating.
2. Description of Related Art
Hitherto, in the field of pinch detection for vehicles, there has been known a pinch detector comprising a worm provided around a rotary shaft of a motor, a worm wheel engaging with the worm, an output shaft for transmitting the rotation of the motor to the outside, a plate member which turns in a body with the output shaft and a spring disposed between the plate member and the worm wheel.
When the worm provided around the rotary shaft of the motor turns, the worm wheel turns along with the turn of the worm, the plate member is pressed by the urging force of the spring disposed within the worm wheel and the output shaft turns, in a body with the plate member. Thus, this apparatus actuates an external apparatus such as a window regulator for opening/closing a glass window or a sun-roof opening system.
For instance, Japanese Utility Model Publication No. Hei. 7-18864 discloses a pinch detector in which the spring flexes and causes relative rotation between the worm wheel and the output shaft when the turn of the output shaft connected to the external apparatus is stopped, while the motor is operative, due to pinching of an object or the like, i.e., when pinch occurs.
Further, as another pinch detector, Japanese Patent Laid-Open No. Hei. 4-134723 discloses an apparatus in which hard rubber is provided between the worm wheel and the output shaft. This causes relative rotation between the worm wheel engaging the worm provided around the rotary shaft of the motor and the output shaft connected to the sun-roof opening system when a pinch occurs. When the output shaft stops while the motor is operative, the rubber functions as a buffer member for buffering a turning force of the motor. Thus, it prevents the teeth of the worm and the worm wheel from chipping in the engagement when the rotation of the motor is locked and when the motor torque is large, including the case when a pinch occurs.
However, although the spring has a linear stress characteristic and can cause the relative rotation between the worm wheel and the output shaft through the urging force thereof when the pinch occurs, regardless of changes of temperature, as can other springs, as shown, for example, in Japanese Utility Model Publication No. Hei. 7-18864, springs do not have enough energy absorbing ability for suppressing the rotation of the motor and cannot therefore be used as a buffer member when the pinch occurs.
Thus, although the turning force of the motor may be stably absorbed when the pinch occurs, or when the motor is locked, by using the spring in the pinch detector, the spring is unable to deal with creep fracture of the tooth surface between the worm and the worm wheel.
Further, although it is possible to fully absorb the turning force of the motor when rubber is used as shown in Japanese Patent Laid-Open No. Hei. 4-134723, because the rubber is hard it exhibits hysteresis (non-linear characteristics) peculiar to rubber when it is stretched/contracted (when resilient/compressed) and the reaction force of the rubber decreases from its original reaction as time elapses (fatigue of rubber).
Accordingly, when rubber is used in the pinch detector, the force applied to the tooth surface decreases when successive force is applied to the tooth surface between the worm and the worm wheel. Therefore, although it is possible to prevent the creep fracture of the tooth surface when the worm wheel used is made of resin because the force applied to the tooth surface decreases, the reaction force thereof becomes unstable generally immediately after being released after having once been compressed, as when a pinch occurs for example. That is, the reaction force is unstable when the window of a vehicle in the closed state is opened and then immediately closed.